Field-Free Switching of Perpendicular Magnetization Induced by Longitudinal Spin-Orbit-Torque Gradient

Spin-orbit-torque (SOT) symmetry in a heavy-metal--ferromagnet bilayer forbids the deterministic switching of perpendicular magnetization. Traditionally, an external magnetic field-exchange bias-tilted magnetic anisotropy is introduced to break the symmetry, which is not suitable for practical applications. Here, we propose to break the SOT symmetry by introducing a longitudinal composition gradient in $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pt}$ heavy metal. We demonstrate the deterministic switching of a perpendicularly magnetized $\mathrm{Co}/\mathrm{Ni}$ multilayer with good endurance when the electric current flows along the direction of the composition gradient. No field-free switching is observed when the current is applied transverse to the gradient or the gradient is removed. The composition gradient of $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{Pt}$ leads to a longitudinal gradient of SOT, which results in the field-free switching of perpendicular magnetization. Our work offers a promising approach for lowering the symmetry of the material system for developing spintronic applications.